[FIX] purchase: Don't auto subscribe recipients on mail composers

Purpose
=======

During the refactoring of the purchase module something went wrong when overriding the 'send_mail' method.

The 'mail_post_autofollow' key should have been set into the context only for the purchase orders model.

See: https://github.com/odoo/odoo/commit/cb01be2#diff-fe8587929089e5fdb6c75687011b1c7eR822

addons/purchase/purchase.py

@@ -1047,7 +1047,8 @@ class MailComposeMessage(models.Model):
     @api.multi
     def send_mail(self, auto_commit=False):
         if self._context.get('default_model') == 'purchase.order' and self._context.get('default_res_id'):
+            self = self.with_context(mail_post_autofollow=True)
             order = self.env['purchase.order'].browse([self._context['default_res_id']])
             if order.state == 'draft':
                 order.state = 'sent'
-        return super(MailComposeMessage, self.with_context(mail_post_autofollow=True)).send_mail(auto_commit=auto_commit)
+        return super(MailComposeMessage, self).send_mail(auto_commit=auto_commit)